Defender is actually pretty underrated - I won't speak for its cleaning abilities because I have no knowledge of them, but it has rather decent detection rates in my experience. Microsoft has the infection statistics and data of Windows PCs all over the globe at their disposal through Defender and their Malicious Software Removal Tool, and detection for widespread ad/spyware like Zlob, rogue antispywares, Storm etc are added promptly from what I can see.